Shown in FIG. 1 is a common mask 10 for patterning, and shown in FIG. 2(a) is amplitude of light on the mask 10, FIG. 2(b) is amplitude of light on a wafer and FIG. 2(c) is intensity of light on the wafer.
Referring to FIG. 1, a common patterning mask 10 includes structure having opaque layers 12 of chrome spaced from each other in a certain interval formed on a transparent substrate 11 of quartz.
As shown in FIG. 2(a), the amplitude of light on the mask 10 overlaps offsetting each other on both edges of the opaque layer 12, and the amplitude, and the intensity of light on the wafer behaves as shown in FIGS. 2(b) and 2(c), respectively.
Therefore, in case the mask 10 is used, it is impossible to obtain a clear patterning due to smaller difference of intensity of light, exhibiting unclear shade, at both edges of the opaque layer 12.
Moreover, because the rate of offset of light on both edges of the opaque layer is greater in a microscopic patterning, a microscopic patterning process can not be carried out with the foregoing mask 10.
Recently, as all elements are highly integrated, requirements on masks for carrying out super microscopic patterning in the order of submicron has risen.
To meet such a requirements, the phase shifting masks have been developed.
Shown in FIG. 3 is a section of a conventional phase shifting mask, and shown in FIG. 4 is wave of light of the phase shifting mask of FIG. 3.
Referring to FIG. 3, a common phase shifting mask 20 includes a structure having opaque layers 22 of chrome spaced from each other in a certain interval formed on a transparent substrate 21 of quartz, between which opaque layers 22 phase shifters 23 are formed.
These phase shifters 23 serve to shift the phase of amplitude of light incident to the substrate.
Shown in FIG. 4(a) is amplitude of light on the phase shifting mask 20 of FIG. 3.
In FIG. 4(a), curve A indicates amplitude of light incident to the substrate 21 when there is no phase shifting mask 23, and curve B indicates amplitude of light incident to the substrate 21 when there is the phase shifter 23.
According to FIG. 4(a), it can be shown that the phase of the amplitude of light incident to the substrate 21 has been shifted 180 degrees by the phase shifter 23.
In FIG. 4(a), the phase difference .delta. between the graph A and the graph B can be represented in following formula (1). ##EQU1##
where, n is the refraction index of the phase shifter 23, d is the thickness of the phase shifter 3, and nO is the refraction index of air.
In formula(1), it can be known that the phase difference should be 180 degrees ie., .pi. for shifting the phase of an amplitude of light completely.
When the phase difference .delta. is substituted by .pi. in formula(1), the thickness of the phase shifter 23 for shifting the phase completely can be represented in following formula. ##EQU2##
A phase shifting mask 20 having conventional phase shifter 23 includes the phase shifter 23 formed between two adjacent opaque layers 22, which phase shifter 23 shifts the phase of incident light onto a substrate 21 180 degrees as shown in FIG. 4(a).
Accordingly, though the phase shifting mask 20 is used, overlap of light at both edges of the opaque layers 23 does not occur, which makes light having an amplitude as shown in FIG. 4(b) incident onto a wafer.
Therefore, using the foregoing phase shifting mask 20 is advantageous for super microscopic patterning due to the great difference of intensity of light at both edges of the opaque layers 23 providing clear shade of light incident onto a wafer.
There are spatial frequency modulation type, edge emphasis type and cut-off effect emphasis type in kinds of phase shifting masks.
Of the foregoing phase shifting masks, a spatial frequency modulation type phase shifting mask has, as known well, disadvantage of carrying out removing unnecessary bridge pattern film formed at the edges of the phase shifter at the time of fabrication.
Recently, as a solution for this problem, a process for fabricating a phase shifter which can shift the phase at the edges of a phase shifter by 90 degrees instead of shifting 180 degrees, is under development.
In a phase shifting mask having a 180 degrees area for shifting the phase at a main area of the phase shifter by 180 degrees, and a 90 degrees area for shifting the phase at edges of the phase shifter by 180 degrees, the phase shifter has to have different thicknesses in the 180 degrees area and the 90 degrees area from each other in order for a phase shifter, shifting phase 180 degrees, to shift the phase 90 degrees only at the edges thereof.
As for the method for fabricating a phase shifting mask having 180 degrees area and 90 degrees area, there is a method for fabricating a phase shifter having different thicknesses in 90 degrees area and 180 degrees area from each other through two times of deposition processes and two times of patterning processes.
The foregoing method is described in detail in SPIE Vol. 1604, 11th Annual BACUS symposium on Photomask Technology, 1991, pp 265 to 273.
However, such a method is cumbersome due to two times of deposition processes in forming the phase shifter and two times of photo etching processes in forming the 90 degrees area and the 180 degrees area.
As an another method, there is a method for fabricating a phase shifting mask having 90 degrees area at the edges of the 180 degrees area by forming 90 degrees area through etching of the edges of the 180 degrees area after a phase shifter of 180 degrees area having been formed.
Referring to FIG. 5, the foregoing Levenson type phase shifting mask is to be explained in detail.
FIGS. 5(a) to 5(c) show a structure of a conventional Levenson type phase shifting mask, of which FIG. 5(a) shows a plan view of the phase shifting mask, FIG. 5(b) shows a section across line A--A' of FIG. 5(a), and FIG. 5(c) shows a section across line B--B' of FIG. 5(c).
Referring FIGS. 5(a) to 5(c), a conventional Levenson type phase shifting mask 30 is provided with a structure having a plurality of chrome patterns 34 formed spaced from each other on a quartz substrate 31 as opaque layers and a plurality of phase shifters 37 formed between and partially overlapped with adjacent two chrome patterns 34 on the substrate 31.
Each phase shifter 37 has 180 degrees area 37-1 and 90 degrees area 37-2, wherein the part corresponding to the opaque layer pattern 34 is 180 degrees area 37-1, and the portion corresponding to the edge parts of the opaque layer pattern 34 is 90 degrees area 37-2.
Referring to FIGS. 6 and 7, a method for fabricating a conventional Levenson type phase shifting mask having a structure as shown in FIG. 5 is to be explained hereinafter.
FIGS. 5(a) to 6(m) and 7(a) to 7(m) show a method for fabricating a conventional Levenson type phase shifting mask, of which, FIGS. 6(a) to 6(m) are sections across line A--A' of FIG. 5(a) showing the fabrication method, and FIGS. 7(a) to 7(m) are sections across line B--B' of FIG. 5(a) showing the fabrication method.
First, as shown in FIGS. 6(a) and 7(a), a chrome layer 32 is formed on a quartz substrate 31, and a negative photoresist film 33 is coated thereon.
Through exposure (FIGS. 6(b) and 7(b)) and development (FIGS. 6(c) and 7(c)), the negative photoresist film 33 is patterned with a predetermined pattern.
Then, the chrome layers are etched using the patterned negative photoresist film 33 (FIGS. 6(d) and 7(d)), and the remained photoresist film 33 is removed to form a plurality of opaque layer patterns 34 (FIGS. 6(e) and 7(e)).
The plurality of opaque layer patterns 34 are positioned in array with a certain interval from each other in the direction of the width thereof (direction of line A--A').
Next, an insulation film 35 is coated on all over the opaque layers 34 (FIGS. 6(f) and 7(f)), on which insulation film 35 a negative photoresist film 36 is coated (FIGS. 6(g) and 7(g)), which is subjected to patterning through exposure (FIGS. 6(h) and 7(h)) and development (FIGS. (7i) and 7(i)) using predetermined pattern.
The insulation film is etched using the patterned negative photoresist film 36 (FIGS. 6(j) and 7(j)), and the remained photoresist film 36 is removed forming a plurality of phase shifters 37 between and partially overlapped with two adjacent opaque layer patterns 34 (FIGS. 6(k) and 7(k)).
This phase shifter 37 is a 180 degrees phase shifter.
Then an etching process is carried out for etching the edges of the phase shifters 37 to a predetermined thickness so as to form the 90 degrees area shifting the phase 90 degrees at the edges of each phase shifters.
Again, a negative photoresist film 38 is coated on all over the surface, which is undertaken a patterning with predetermined pattern exposing both edges of the phase shifters 37 in the longitudinal direction(direction of line B--B' of FIG. 5(a) thereof (FIGS. 6(l) and 7(l)).
The exposed edge part of the phase shifter 37 is etched to a certain thickness using the patterned negative photoresist film 38.
Thus, fabrication of a conventional phase shifting mask is completed by forming phase shifters 37 which can shift the phase 90 degrees at both edges not overlapping with the opaque layer patterns 37 and shift the phase 180 degrees in rest of the part.
As such, the foregoing method for fabricating a conventional Levenson type phase shifting mask has a cumbersome process of carrying out photo etching process once more to form the 90 degrees area 37-2 which shifts the phase 90 degrees at edges of the phase shifters, and a problem of difficulty in controlling the process for etching the phase shifters of the edge part to shift the phase 90 degrees exactly.